Additives are added to synthetic resinous materials to provide products having improved performance (mechanical, chemical) and/or physical characteristics (color, surface texture); reduced sensitivity to degradation by heat, light and chemicals; lower cost; and other desired attributes. Typically, an additive is “polymer-and purpose-specific”, that is, a particular additive is chosen for a particular purpose in a specific polymer. When the additive itself is in a fluid state and not a solid, addition of the additive is simplified as it can be conventionally added to virgin polymer so long as the degradation temperature of the additive is below the fluidization temperature of the polymer. By “virgin” polymer is meant, polymer which is commercially manufactured for melt-processing into articles of arbitrary shape and size. Such virgin polymer may be a single polymer having a wide range of molecular weight, or a blend of polymers and may include such polymers which are recycled.
When the additive is a particulate, finely divided solid, dispersing the additive uniformly through additive-enriched (or “additive-rich” for brevity) melt to yield a homogeneous melt is difficult, the smaller the particles, the greater the difficulty. Such difficulty is compounded when the particles are smaller (have an equivalent diameter) than 45 μm (325 mesh), and/or thermally degradable at the fluidization temperature of the polymer, and particularly so if they are required to be uniformly dispersed in the melt, and remain uniformly dispersed after the additive-rich melt is cooled and the particles try to revert to an equilibrium state. One skilled in the art is all too well aware of the difficulties of wetting, separating and stabilizing any powdery additive so it is substantially homogeneously dispersed in a solid polymer.
By “substantially homogeneously dispersed” or “uniformly dispersed” or “uniformity of dispersion” is meant that the uniformity of dispersed particles in the film may be quantified by known microscopic techniques, X-ray diffraction (XRD) analysis, or by a blown film test. In the blown film test, the polymer containing solid powder particles is extruded through a blown film apparatus which produces a thin enough film to be transparent to visible light, e.g. about 0.025 mm (1 mil) thick, and this film is placed over a light source of appropriate wavelength and intensity to enable one to quantify the number of particles which show up as “imperfections”; and the size of each is also visible under appropriate magnification. For particles in a concentration greater than about 8% by wt, “filled” melt is placed at one end of a glass slide and smeared across it with a thin blade.
Dispersion of particles is typically deemed uniform by one skilled in the art when the distribution of particles in one unit area is visually essentially the same as that in a neighboring unit area and throughout the film, and each unit area is substantially free from visually evident agglomerates. More specifically the number of particles per unit area is within about 20% or less of the average number of particles in each other unit area in the film, and the interparticle spacing per unit area is within 20% or less of the average spacing in each other unit area in the film, allowance being made for disparities in particle size. One skilled in the art knows uniformity of dispersion when he sees it.
For “nanoparticles” in the range from 1 nm (nanometer) but less than 1 μm (micrometer), e.g. nanoclay particles, the extent of intercalation and exfoliation in the polymer matrix may be determined by XRD analysis, as illustrated below. Nanoclays are typically lightweight organoclay platelets that are 200-600 nm in width and length, and 1 nm thick. See Appliance, August 2004, v 61 i8 p34(1).
The “melt” processed herein refers either to a single polymer or a miscible blend of two or more polymers at or above the fluidization temperature of the polymer or blend, and each polymer may be crystalline, partially crystalline or amorphous. The melt temperature of a substantially crystalline polymer or blend is that temperature at which the polymer or blend melts, typically not sharply, but over a narrow range, at ambient pressure. The “melt” temperature at which a substantially amorphous polymer or blend begins to flow is its “melt-controlling temperature”, that is, the highest glass transition temperature Tg, if the amorphous polymer has more than one Tg. At, or above the fluidization temperature the polymer is said to be “melt-processable”. The term “melt” is used herein to indicate fluidized melt-processable polymer at a fluidization temperature which is at least as high as the polymer's “melt” temperature, and in practice, at least 10° C. higher, typically from 20° C. to 75° C. higher.
The Problem:
The difficulty of dispersing very small particles, particularly nanoclay platelets, to obtain a uniform dispersion, maximum intercalation and exfoliation, to the extent that it can be, stems from increasing the shear which requires lowering the temperature to achieve the desired delamination effect. However, as the viscosity and shear increase, the flow equation requires that pressure increase at least 10%. This pressure increase opposes the dispersing effect of the shear delamination. Increasing the temperature to release pressure, forces one against the limitation of degradation of both the resin and the additive. As a result, in conventional dispersions of nanoclays with the strictures of the foregoing considerations, there is still present a substantial and easily detectable population of relatively large agglomerates, one does not expect to find in a matrix in which primary particles of the additive are uniformly distributed. For obvious reasons, the difficulty of obtaining substantially uniform dispersions of micron-sized and nanoparticles is exacerbated when the additive to be dispersed is thermally sensitive.
How to modify the physical and physico-chemical characteristics of a polymer, and how to make a “stress-fatigued” melt which is fluidizable at a temperature below the virgin polymer's conventional fluidization temperature, is disclosed in U.S. Pat. Nos. 4,469,649; 5,306,129; 5,494,426; 5,885,495; and 6,210,030 issued to Ibar. In the '495 process, virgin polymer, that is, polymer conventionally manufactured and purchased in the market place, is extruded to form a melt which is then led into an apparatus referred to as a TekFlow® processor, available from Stratek Plastic Ltd. (Dublin, Ireland) and SPRL Inc. (Wallingford, Conn., USA). The melt is mechanically vibrated and fatigued until the state of entanglement between the molecules has been modified to a desired level of disentanglement as measured by a decrease of at least 10% in the viscosity and melt modulus of elasticity relative to that of the virgin melt after correction of the influence of degradation of the chains, on viscosity. The resulting polymer, referred to herein as being “disentangled”, “extensively shear-thinned”, or “stress-fatigued” is referred to herein as “modified” polymer melt (for brevity), and is characterized by having a fluidization temperature at least 10° C. lower than the fluidization temperature of the same virgin polymer had it not been extensively shear-thinned and stress-fatigued.
The '495 patent states: “Yet, in another embodiment of the present invention, the vibrated melt per the present invention is extruded or co-extruded with other melts and additives, and pelletized just after the vibration treatment is performed to obtain solid granules or pellets of the treated melt. The extrusion is done in a way which minimizes the recovery process to take place, for example, under minimum pressure in the case the vibration treatment reduced the viscosity of the melt by extensional shear to reduce the entanglements, and conversely, under minimum shear in the case the vibration treatment increased the elasticity of the melt by favoring the interpenetration of the macro-molecules and increasing the entanglements.” (see '495, col 6, lines 12-24).
It is evident that in a two-step, non-continuous process, one may disperse a thermally sensitive additive in a melt after first determining if the processor could be operated to produce a modified melt with a fluidization temperature lower than that at which the proposed additive would be degraded. In this first step, fluid polymer melt from a polymer-melting means such as a conventional extruder, is extruded into the processor, and the melt is cooled and pelletized. In the second step, if the fluidization temperature of the modified melt can be lowered to, preferably below, the degradation temperature of the additive, the additive is mixed with the pelletized, modified polymer, and melt-processed, e.g. extruded through a conventional extruder, to produce an additive-rich melt which is cooled to form a solid stress-fatigued polymer with the additive dispersed in it. In this two-step process, though the additive is thermally degradable at the fluidization temperature of the virgin polymer, it will be evident that the additive would not be degraded at the fluidization temperature of the modified melt because it would typically be at a temperature below the degradation temperature after being processed in the processor.
Referring to the patents identified above, there is no provision for introducing any material into virgin polymer being stress-fatigued in the apparatus (which fatigues the melt) after the virgin polymer is fed into the feed-inlet of the apparatus such as a TekFlow® processor, and there is no suggestion in the disclosures of the patents as to how any material may be added to the melt within the processor.
Nanoclays are typically converted to organoclays to lessen the difficulty of dispersing the platelets into a melt to form a nanocomposite. Thermal sensitivity of organoclays is usually attributable to a chemically attached binder or compatibilizing agent to improve dispersibility. These clays are nevertheless particularly difficult to substantially fully intercalate and exfoliate. See “Phase Morphology and Rheological Behavior of Polymer/Layered Silicate Nanocomposites,” by Lim, et al., Rheol Acta 4: 220-229 (2001). Natural montmorillonite nanoclay, modified/coated with a quaternary ammonium salt (available from Southern Clay Products) improves stiffness, heat deflection temperature and barrier properties of the matrix in which the clay is distributed. The modified nanoclay increases intercalation (D001 spacing) to facilitate mechanical exfoliation (i.e. separation of platelets) and dispersion of the nanoclay in a melt. The thermal stability of nanoclays, depending upon the substituent introduced, begins to decrease above about 200° C.
The extent of degradation may be determined by FT-IR spectroscopy which shows the thermal degradation of the onium compound present in the clay. Though the dispersibility and exfoliation, particularly in an aqueous environment, is improved by an organophilic substituent, there is no suggestion in the art that a nanoclay in an amount greater than 10% by weight of the additive-rich matrix, and more particularly in a range from about 10% to 30% may be homogeneously dispersed in a polymer, yet be mechanically substantially fully exfoliated. Concentrations stated as “% by weight” refer to the amount in the additive-enriched polymer.